Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M 1 = 0.38 ± 0.02 M and a radius of R 1 = 0.0222 ± 0.0004 R. The secondary white dwarf's mass and radius are M 2 = 0.32 ± 0.01 M and R 2 = 0.0207 ± 0.0004 R, and its temperature and surface gravity (T 2 = 11678 ± 313 K, log(g 2) = 7.32 ± 0.02) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5 per cent relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of ≤10−4 M *. The two stars will merge in 1.14 ± 0.07 Gyr due to angular momentum loss via gravitational wave emission.